During December 1941, Roosevelt devised the name "United Nations" for the Allies, and the Declaration by United Nations, on 1 January 1942, was the basis of the modern UN.[3] At the Potsdam Conference of July-August 1945, Roosevelt's successor, Harry S. Truman, proposed that the foreign ministers of China, France, the Soviet Union, the United Kingdom and the United States "should draft the peace treaties and boundary settlements of Europe," which led to the creation of the Council of Foreign Ministers.[4]

During the 1920s, the Kuomintang (KMT) government led by Generalissimo Chiang Kai-shek was aided by the Soviet Union, which helped to reorganise the party, superficially at least, along Leninist lines: a unification of party, state, and army. However, following the nominal unification of China in 1928, Chiang Kai-shek purged leftists from his party and fought against the Chinese Communist Party, former warlords, and other militarist factions. A fragmented China provided easy opportunities for Japan to gain territories piece by piece without engaging in total war. Following the 1931 Mukden Incident, the puppet state of Manchukuo was established. Throughout the early to mid 1930s, Chiang's anti-communist and anti-militarist campaigns continued while he fought small, incessant conflicts against Japan, usually followed by unfavorable settlements and concessions.

Even though China had been fighting the longest among all the Allied powers, it only officially joined the Allies after the attack on Pearl Harbor, on 7 December 1941. Chiang Kai-shek felt Allied victory was assured with the entrance of the United States into the war, and he declared war on Germany and the other Axis nations. However, Allied aid remained low because the Burma Road was closed and the Allies suffered a series of military defeats against Japan early on in the campaign. The bulk of military aid did not arrive until the spring of 1945. More than 1.5 million Japanese troops were trapped in the China Theatre; troops that otherwise could have been deployed elsewhere if China had collapsed and made a separate peace with Japan.

Following the Statute of Westminster in 1931, the Dominions of the British Commonwealth had independence in foreign policy. Australia and New Zealand accepted and reiterated the British declaration. Nepal, another independent Kingdom, declared war against imperialism on 4 September. The South African Prime Minister, Barry Hertzog, refused to declare war, leading to the collapse of his coalition government on 6 September; the new Prime Minister, Jan Smuts, declared war that same day. Canada declared war on Germany on 10 September; this was necessary as Canada had ratified the Statute of Westminster.

The United States of America joined the Allies following the attack on Pearl Harbor, on 7 December 1941. The Declaration by United Nations, on 1 January 1942, officially united 26 nations as Allies. The informal Big 3 alliance of the United Kingdom, the Soviet Union, and the United States emerged in the later half of the war, and their decisions determined Allied strategy around the world.

These countries were allied to each other by a net of common defence pacts and military alliance pacts signed before the war. The Franco-British Alliance dated back to the Entente Cordiale of 1904 and the Triple Entente of 1907, active during the World War I. The Franco-Polish Alliance was signed in 1921 and then amended in 1927 and 1939. The Polish-British Common Defence Pact, signed on 25 August 1939, contained promises of mutual military assistance between the nations in the event either was attacked by Nazi Germany.

Apart from these were the independent members of the British Commonwealth known as the Dominions. These declared war on Germany separately, either on the same day, or soon afterwards. These countries were: Australia, Canada, New Zealand and South Africa. Newfoundland had given up self-rule and was at the time under effective rule from the UK; it did not become part of Canada until 1949.

The British Indian Empire contributed about 2,500,000 personnel. It suffered 1,500,000 civilian casualties (more than the United Kingdom), mainly from the Bengal famine of 1943 caused by the fall of Burma to the Japanese,[9] and 87,000 military casualties (more than any Commonwealth country but fewer than the United Kingdom). The UK suffered 382,000 military casualties.

Denmark was invaded by Germany on 9 April 1940. The Danish government did not declare war and it surrendered the same day, on the understanding that it retain control of domestic affairs. No government-in-exile was formed. Danes fought with both Allied and Axis forces. Iceland, Faroe Islands and Greenland, which were respectively in union with Denmark and a Danish colony, were occupied by the Allies for most of the war. British forces took control in Iceland on 10 May 1940, and it was used to facilitate the movement of Lend Lease equipment. Forces from the United States, although they were officially neutral at the time, occupied Greenland on 9 April 1941. The U.S. also took over in Iceland on 7 July 1941. Iceland declared full independence from Denmark in 1944, but never declared war on any of the Axis powers.

Although Portugal remained officially neutral, and the Salazar Dictatorship admired Fascist regimes, there was the Anglo-Portuguese Alliance — the world's oldest military alliance (1373) — reactivated by the United Kingdom during World War II, leading to the establishment of an Anglo-American base in Lajes, Terceira Island, Azores, which Salazar finally accepted (December 1943), though he was not in position to refuse anyway. Since 1940, both Churchill and Roosevelt were facing the possibility of a preventive occupation of Azores.[11] Portugal also protested the occupation of Portuguese Timor by Allied forces in 1942 but did not actively resist. The colony was subsequently occupied by Japan. Timorese and Portuguese civilians assisted Australian commandos in resisting the Japanese.

The members of the Pan American Union, who were all neutral between 1939 and 1941, formed a mutual defense pact at a conference of foreign ministers at Havana, on 21 July 1940 – 30 July 1940. The "Declaration on Reciprocal Assistance and Cooperation for the Defense of the Nations of the Americas" was part of the Final Act of the Second Meeting of the Ministers of Foreign Affairs of the American Republics at Havana, Cuba, July 30, 1940.[12] There were twenty-one signatories:

From this group, three countries participated in the War taking military actions against the Nations of Axis Powers:

USA, as one of the Big Three, the leader group of Allies Nations (at the side of the British empire - which was led by United Kingdom - and the then USSR) played the major role in defeat and surrender of Japan as well as a decisive ones in defeat of Fascist Italy and Nazi Germany;

The alliance was formalised in the Declaration by United Nations on 1 January 1942. There were 27 signatories, as follows:

Australia

Belgium

Brazil

Republic of China

Canada

Colombia

Costa Rica

Cuba

Czechoslovakia

Dominican Republic

El Salvador

Greece

Guatemala

Haiti

Honduras

India

Luxembourg

Mexico

Netherlands

New Zealand

Nicaragua

Norway

Panama

Peru

Philippines

Poland

South Africa

United Kingdom

Union of Soviet Socialist Republics

United States

Yugoslavia

Later in 1942, Mexico, Philippine Commonwealth and Ethiopia adhered to the declaration. During 1943, it was signed by Iraq, Iran, Brazil, Bolivia and Colombia. In 1944, Liberia and France signed . During the early part of 1945, Peru, Chile, Paraguay, Venezuela, Uruguay, Turkey, Egypt, Saudi Arabia, Lebanon, Syria and Ecuador became signatories.

On 29 January 1946, the United Kingdom and the Soviet Union agreed to end their occupation of Iran, six months after the end of the war. The Tripartite Treaty of Alliance also formalised Iran's assistance to the Allies.[14]